Antenna diversity is a technique used in antenna-based communication systems, such as, e.g., cellular radio, to reduce the effects of multi-path distortion fading. Antenna diversity may be obtained by providing a receiver with two or more (i.e., N.gtoreq.2) antennas. These N antennas imply N channels which suffer fading in a statistically independent manner. Thus, when one channel is fading--that is, suffering amplitude loss due to the destructive effects of multi-path interference, another of these channels is unlikely to be suffering from fading simultaneously. By virtue of the redundancy provided by these independent channels, a receiver can often avoid the detrimental effects of fading.
Receivers of broadcast transmissions, of course, require at least one antenna to receive a broadcast signal. Each additional antenna used to facilitate the avoidance of fading adds to receiver cost and complexity. Naturally, it is desirable to avoid the effects of fading using as few antennas as possible.
In time division duplex (TDD) radio transmission systems, mobile radio units equipped with a single antenna can obtain the benefits of antenna diversity if the associated base station employs multiple antennas when transmitting information. Antenna diversity is possible in TDD systems using the principle of adaptive retransmission so long as system channel response characteristics (e.g., amplitude and fade) in both transmission directions are essentially the same. See W. C. Jakes, Jr., Ed., Microwave Mobile Communications (1974).
Despite the fact that TDD-based systems can provide antenna diversity using a single antenna at the receiver, the desirability of such systems for mobile communication, such as cellular radio, is offset by a number of factors. First, TDD-based systems require precise clock synchronization across base stations. Without such clock synchronization, a mobile-to-base transmission in one cell can be substantially interfered with by a base-to-mobile transmission in a neighboring cell using the same frequency. Although suitable clock synchronization may be provided through use of the global positioning system (GPS), GPS-based timing adds significantly to base-station (and thus user) cost.
Second, for TDD systems with slow signaling rates (such as those specified by IS-54, a narrow-band TDMA digital cellular standard), the base-to-mobile and mobile-to-base channels may be different thereby undermining the benefits of adaptive retransmission. Although the problem of slow signaling rate can be addressed through the use of shorter time slots, this can result in significant loss of capacity if fixed signaling overhead is required for each slot.
Third, and perhaps most importantly, TDD communication systems are incompatible with existing cellular systems, which employ frequency division duplex (FDD) communication.
FDD-based communication systems do not present the timing and compatibility difficulties discussed above with reference to TDD systems. However, FDD-based systems have no capability to perform adaptive retransmission since mobile-to-base and base-to-mobile transmission occurs in two independently fading frequency bands. A diversity benefit in FDD-based systems using a single receiving antenna has been provided in the past by the repeated transmission of a set of information symbols over one channel at two distinct intervals in time. Because only one channel is actually in use, diversity is provided by separating the two transmissions far enough in time so that the fading characteristics of the channel can change in the interim. Assuming a change in channel fading characteristics, this technique simulates the use of two independently fading channels, and a diversity benefit may therefore be provided.
Unfortunately, in order be reasonably certain of a change in fading characteristics between transmissions, a significant inter-transmission delay is required. This in turn delays the ultimate determination of the transmitted information by the mobile unit.